I’ve just turned 56 and still feel much like I did in my 30s, in some ways slightly better from improved nutrition and more consistent exercise.
I do notice a few more niggling aches and pains and recovery is a little slower, but my aerobic fitness is as good as it ever was and I have actually dropped a few kg in weight without losing any muscle. I’m certainly fitter than I was 10 years ago in my mid 40s when I had dropped off a fair bit from my early 30s.
I take the view that lifestyle (especially exercise) trumps age, at least for now. But maybe it takes a little more effort to retain fitness. I could get away with a lot of bad habits in my 20s that would really take their toll in my 50s. For example I need quality sleep and nutrition now. In my 20s I could do all-nighters and metabolise pretty much any junk food!